1. Field of the Invention
The present invention relates to a keyswitch assembly and, more particularly, to a keyswitch assembly suitable for use on a thin keyboard for a portable word processor, a portable personal computer or the like.
2. Description of Related Art
A known keyswitch assembly for use on such a keyboard has a key provided with a stem, a base plate provided with a key support having a hole receiving the stem of the key to guide the key for vertical movement, and a switching member, such as a membrane switch, disposed under the stem. When the key is depressed, the lower end of the stem of the key presses the switching member for switching action.
A keyswitch assembly provided with a large key, such as a space key and a return key, is provided with a mechanism for maintaining the key in a level position when the key is depressed. Such keyswitch assemblies are disclosed in U.S. Pat. Nos. 4,580,022, 4,902,862 and 4,433,225.
In a keyswitch assembly disclosed in the '022 patent, a key member is supported on support levers connected with pins in a scissors-like form, and switching members are disposed apart from the central portion of the key member. Pins attached to the opposite ends of the support levers slide horizontally along the inner surface of the key member and the upper surface of a base plate when the key member is depressed. Stems formed in the key member and guided by a guide member slide vertically to compress the switching members when the key member is depressed.
A keyswitch assembly disclosed in the '862 patent is the same in basic construction as the keyswitch assembly disclosed in the '022 patent and is characterized in that the key member can be easily connected to and removed from the support levers.
In the '225 patent, a keyswitch assembly including an L-shaped keytop is disclosed. The keyswitch assembly comprises a pair of lever arms joined at intermediate portions thereof by a pivot to form a scissors-like linkage having first, second, third, and fourth ends. The first and second ends of the scissors-like linkage are pivotally slidable within the cantilevered portion of the keytop. However, a keyswitch portion is separately disposed from the scissors-like linkage. So, there is a problem that the keyswitch is not perfectly operated. Furthermore, a plunger is needed, so the assembly requires many parts, and the structure is complex.
In these prior art keyswitch assemblies disclosed in the above references, the key member is maintained in a level position regardless of the position at which pressure is applied to the key member, when the key member is moved vertically, even if the key member is a large key member, such as a space bar.
In any of these prior art keyswitch assemblies, the stem for compressing the switching member or the key depressing member is guided for vertical sliding movement by the guide member in compressing the switching member.
Recent progressive reduction in size and thickness of word processors and personal computers requires reduction in size and thickness of keyboards to be incorporated into word processors and personal computers. On the other hand, the stroke of the keys of keyboards must be sufficiently large to facilitate keystroke operation and to secure a reliable keystroke. However, the stroke of the keys of the prior art keyswitch assemblies, which are generally used, is not sufficiently large.
In reducing the thickness of a keyboard provided with the generally used prior art keyswitch assemblies, the length of a sliding portion of the stem in sliding engagement with the guide member must be reduced. However if the length of the sliding portion of the stem is reduced, the key is liable to tilt relative to the guide member and, consequently, the stem is liable to slide awkwardly in the guide member when the key is depressed. If the length of the sliding portion of the stem in engagement with the guide member is increased to ensure smooth movement of the key, the stroke of the key is reduced. Thus, the effort of reducing the thickness of the keyboard by reducing the sliding portion of the stem and the effort of securing smooth movement of the key are contradictory.
If the stem of the key slides awkwardly in the guide member when the key is depressed, noise is generated and the ease of the keystroke operation is deteriorated. However, the misalignment of the stem with the guide member does not occur very of ten if the key is always depressed in the central portion thereof. Hence, it may be possible to obviate the misalignment of the stem with the guide member by forming the upper surface of the key as a small area so that the key is always depressed in the central portion thereof, which also will deteriorate facility in keystroke operation.
The keyswitch assemblies disclosed in the foregoing references are not intended to enable the reduction of the thickness of the keyboard and, since the stem for compressing the switching member must be projected from the key, it is difficult to form those keyswitch assemblies in a relatively small thickness. Further, the key provided with the stem has a complicated shape and hence increases the cost of the keyswitch assemblies.
Furthermore, since the pivots formed at the extremities of the support levers slide horizontally along the inner surface of the key and the upper surface of the base plate, respectively, the position of the key with respect to horizontal directions is indefinite. Consequently, it is possible that the key is dislocated horizontally when depressed, and the stem is unable to operate the switching member reliably.
In particular in the '225 patent, the keyswitch is poorly operated because the keyswitch is separately disposed from the scissors-like linkage. In addition, there is the problem that many parts are needed because such a keyswitch assembly requires a plunger.